Augmented digestion of lignocellulose by steam explosion, acid and alkaline pretreatment methods: a review

Effect of steam explosion pretreatment on the anaerobic digestion of rice straw

Steam explosion pretreatment on the anaerobic digestion of rice straw.

Properties of Cellulose Microfibers Extracted from Pineapple Leaves by Steam Explosion

The aim of this work is to compare properties of cellulose mats from pineapple leaf fibers with and without using the steam explosion method. Pineapple leaf fibers were divided into two groups. The first group was chemically treated with sodium hydroxide for 24 h, and directly used to prepare cellulose mats. The other group was pre-treated by steam explosion before treating with sodium hydroxide. Cellulose mats of microfibers were then prepared. The structure of pineapple leaf fibers was found to be changed due to the steam explosion observed by scanning electron microscopy. The effect of the steam explosion treatment was studied from chemical composition of steam-exploded fibers and unsteam-exploded fibers. Mechanical properties of mats of steam-exploded fibers were also investigated, compared to those of unsteam-exploded fiber mats. Mechanical properties of the mats of steam-exploded fibers were higher than those of the mats of unsteam-exploded fibers. This is due to the fact that the smaller-size fibrils can be found in the steam-exploded fibril mats.

Optimization of the steam explosion and enzymatic hydrolysis for sugars production from oak woods

Fermentable sugars production from three kind of steam-exploded oak wood was optimized by Response Surface Methodology (RSM), using the severity factor (R0), the pretreated total solids (TS%) and the enzyme loading (EL%) as variables of a central composite design. A total of 17 experiments for each biomass were carried out. The optimal conditions established with RSM were: severity, 4.46 for holm, 4.03 for turkey oak and 3.92 for downey oak; total solids, 5.4% for holm, 5.0% for turkey oak and 12.7% for downey oak; and enzyme concentration, 9.6% for holm, 15.0% for turkey oak and 15.0% for downey oak. Under these conditions, the model predicted an overall sugar yield of 67.1% for holm, 79.9% for turkey oak and 68.4% for downey oak. The results of the confirmation experiments under optimal conditions agreed well with model predictions. Oak wood species may be a good feedstock for the production of reducing sugars.

Assessment of Shock Pretreatment of Corn Stover Using the Carboxylate Platform

Corn stover was pretreated with lime and shock, a mechanical process that uses a shockwave to alter the biomass structure. Two pretreatments (lime-only and lime + shock) were evaluated using enzymatic hydrolysis, batch mixed-culture fermentations, and continuous countercurrent mixed-culture fermentation. In a 120-h enzymatic hydrolysis, shock pretreatment increased the glucan digestibility of submerged lime pretreatment (SLP) corn stover by 3.5 % and oxidative lime pretreatment (OLP) corn stover by 2.5 %. The continuum particle distribution model (CPDM) was used to simulate a four-stage continuous countercurrent mixed-culture fermentation using empirical rate models obtained from simple batch experiments. The CPDM model determined that lime + shock pretreatment increased the total carboxylic acids yield by 28.5 % over lime-only pretreatment in a countercurrent fermentation with a volatile solids loading rate (VSLR) of 12 g/(L/day) and liquid retention time (LRT) of 30 days. In a semi-continuous countercurrent fermentation performed in the laboratory for 112 days with a VSLR of 1.875 g/(L day) and LRT of 16 days, lime + shock pretreatment increased the total carboxylic acid yield by 14.8 %. The experimental results matched closely with CPDM model predictions (4.05 % error).

Selection of the Strain Lactobacillus acidophilus ATCC 43121 and Its Application to Brewers' Spent Grain Conversion into Lactic Acid

Six Lactobacillus strains were analyzed to select a bacterium for conversion of brewers' spent grain (BSG) into lactic acid. Among the investigated strains, L. acidophilus ATCC 43121 showed the highest yield of lactic acid production (16.1 g/L after 48 hours) when grown in a synthetic medium. It was then analyzed for its ability to grow on the hydrolysates obtained from BSG after acid-alkaline (AAT) or aqueous ammonia soaking (AAS) pretreatment. The lactic acid production by L. acidophilus ATCC 43121 through fermentation of the hydrolysate from AAS treated BSG was 96% higher than that from the AAT treated one, although similar yields of lactic acid per consumed glucose were achieved due to a higher (46%) glucose consumption by L. acidophilus ATCC 43121 in the AAS BSG hydrolysate. It is worth noting that adding yeast extract to the BSG hydrolysates increased both the yield of lactic acid per substrate consumed and the volumetric productivity. The best results were obtained by fermentation of AAS BSG hydrolysate supplemented by yeast extract, in which the strain produced 22.16 g/L of lactic acid (yield of 0.61 g/g), 27% higher than the value (17.49 g/L) obtained in the absence of a nitrogen source.

The correlation between cellulose allomorphs (I and II) and conversion after removal of hemicellulose and lignin of lignocellulose

H2SO4, NaOH and H3PO4 were applied to decompose lignocellulose samples (giant reeds, pennisetum and cotton stalks) to investigate the correlation between cellulose allomorphs (cellulose I and II) and conversion of cellulose. The effect of removal of hemicellulose and lignin on the surface morphology, crystallinity index (CrI), cellulose allomorphs (cellulose I and II), and enzymatic hydrolysis under different pretreatments was also studied. CrI caused by H3PO4 pretreatment reached 11.19%, 24.93% and 8.15% for the three samples, respectively. Corn stalk showed highest conversion of cellulose among three samples, irrespective of the pretreatment used. This accounted for the widely use of corn stalk as the renewable crop substrate to synthesize biofuels like ethanol. CrI of cellulose I (CrI-I) negatively affects cellulose conversion but CrI of cellulose II (CrI-II) positively affects cellulose conversion. It contributes to make the strategy to transform cellulose I to cellulose II and enhancing enzymatic hydrolysis of lignocellulose.

Comparison of industrially viable pretreatments to enhance soybean straw biodegradability

This study explores acid and alkaline pretreatments in order to enhance soybean straw biodegradability. The effects of sulfuric acid and sodium hydroxide for different pretreatment times at 30°C and 121°C on biomass dissolution and the subsequent enzymatic hydrolysis were investigated. The highest total conversion to reducing sugars of 93.9% was attained when soybean straw was pretreated with acid (4% H2SO4, 121°C, 1 h) and subsequently subjected to the enzymatic process. However, conversion of 86.5%, were reached only with the hydrolysis of the pretreated residue using mild conditions, (0.5% NaOH, 30°C, 48 h), involving the reduction cost of the process. In addition to this, this result was dramatically decreased when pectinase was removed from the enzyme cocktail. It has been also demonstrated that the reduction of the enzyme loading to less than half allowed obtaining about 96% of the reducing sugars attained with the highest enzyme dose.

An overview of key pretreatment processes for biological conversion of lignocellulosic biomass to bioethanol

Second-generation bioethanol can be produced from various lignocellulosic biomasses such as wood, agricultural or forest residues. Lignocellulosic biomass is inexpensive, renewable and abundant source for bioethanol production. The conversion of lignocellulosic biomass to bioethanol could be a promising technology though the process has several challenges and limitations such as biomass transport and handling, and efficient pretreatment methods for total delignification of lignocellulosics. Proper pretreatment methods can increase concentrations of fermentable sugars after enzymatic saccharification, thereby improving the efficiency of the whole process. Conversion of glucose as well as xylose to bioethanol needs some new fermentation technologies to make the whole process inexpensive. The main goal of pretreatment is to increase the digestibility of maximum available sugars. Each pretreatment process has a specific effect on the cellulose, hemicellulose and lignin fraction; thus, different pretreatment methods and conditions should be chosen according to the process configuration selected for the subsequent hydrolysis and fermentation steps. The cost of ethanol production from lignocellulosic biomass in current technologies is relatively high. Additionally, low yield still remains as one of the main challenges. This paper reviews the various technologies for maximum conversion of cellulose and hemicelluloses fraction to ethanol, and it point outs several key properties that should be targeted for low cost and maximum yield.

Recent Advances in the Application of Inorganic Salt Pretreatment for Transforming Lignocellulosic Biomass into Reducing Sugars

Currently, the transformation of lignocellulosic biomass into value-added products such as reducing sugars is garnering attention worldwide. However, efficient hydrolysis is usually hindered by the recalcitrant structure of the biomass. Many pretreatment technologies have been developed to overcome the recalcitrance of lignocellulose such that the components can be reutilized more effectively to enhance sugar recovery. Among all of the utilized pretreatment methods, inorganic salt pretreatment represents a more novel method and offers comparable sugar recovery with the potential for reducing costs. The use of inorganic salt also shows improved performance when it is integrated with other pretreatment technologies. Hence, this paper is aimed to provide a detailed overview of the current situation for lignocellulosic biomass and its physicochemical characteristics. Furthermore, this review discusses some recent studies using inorganic salt for pretreating biomass and the mechanisms involved during the process. Finally, some prospects and challenges using inorganic salt are highlighted.

Optimization of uncatalyzed steam explosion pretreatment of rapeseed straw for biofuel production

Rapeseed straw constitutes an agricultural residue with great potential as feedstock for ethanol production. In this work, uncatalyzed steam explosion was carried out as a pretreatment to increase the enzymatic digestibility of rapeseed straw. Experimental statistical design and response surface methodology were used to evaluate the influence of the temperature (185-215°C) and the process time (2.5-7.5min). According to the rotatable central composite design applied, 215°C and 7.5min were confirmed to be the optimal conditions, considering the maximization of enzymatic hydrolysis yield as optimization criterion. These conditions led to a maximum yield of 72.3%, equivalent to 81% of potential glucose in pretreated solid. Different configurations for bioethanol production from steam exploded rapeseed straw were investigated using the pretreated solid obtained under optimal conditions as a substrate. As a relevant result, concentrations of ethanol as high as 43.6g/L (5.5% by volume) were obtained as a consequence of using 20% (w/v) solid loading, equivalent to 12.4g ethanol/100g biomass.

Bioaugmentation with an anaerobic fungus in a two-stage process for biohydrogen and biogas production using corn silage and cattail

Bioaugmentation with an anaerobic fungus, Piromyces rhizinflata YM600, was evaluated in an anaerobic two-stage system digesting corn silage and cattail. Comparable methane yields of 328.8±16.8mLg(-1)VS and 295.4±14.5mLg(-1)VS and hydrogen yields of 59.4±4.1mLg(-1)VS and 55.6±6.7mLg(-1)VS were obtained for unaugmented and bioaugmented corn silage, respectively. Similar CH4 yields of 101.0±4.8mLg(-1)VS and 104±19.1mLg(-1)VS and a low H2 yield (<1mLg(-1)VS) were obtained for unaugmented and bioaugmented cattail, respectively. However, bioaugmentation resulted in an initial increase in CH4 and H2 production rates and also increased volatile fatty acid degradation rate for both substrates. Our study demonstrates the potential of bioaugmentation with anaerobic fungus for improving the digestibility of lignocellulose substrates for biogas and biohydrogen production.

Structural features of dilute acid, steam exploded, and alkali pretreated mustard stalk and their impact on enzymatic hydrolysis

To overcome the recalcitrant nature of biomass several pretreatment methodologies have been explored to make it amenable to enzymatic hydrolysis. These methodologies alter cell wall structure primarily by removing/altering hemicelluloses and lignin. In this work, alkali, dilute acid, steam explosion pretreatment are systematically studied for mustard stalk. To assess the structural variability after pretreatment, chemical analysis, surface area, crystallinity index, accessibility of cellulose, FT-IR and thermal analysis are conducted. Although the extent of enzymatic hydrolysis varies upon the methodologies used, nevertheless, cellulose conversion increases from <10% to 81% after pretreatment. Glucose yield at 2 and 72h are well correlated with surface area and maximum adsorption capacity. However, no such relationship is observed for xylose yield. Mass balance of the process is also studied. Dilute acid pretreatment is the best methodology in terms of maximum sugar yield at lower enzyme loading.

Production of cellulosic ethanol from cotton processing residues after pretreatment with dilute sodium hydroxide and enzymatic hydrolysis

In this study, production of cellulosic ethanol from two cotton processing residues was investigated after pretreatment with dilute sodium hydroxide. Pretreatment performance was investigated using a 2(2) factorial design and the highest glucan conversion was achieved at the most severe alkaline conditions (0.4g NaOH g(-1) of dry biomass and 120°C), reaching 51.6% and 38.8% for cotton gin waste (CGW) and cotton gin dust (CGD), respectively. The susceptibility of pretreated substrates to enzymatic hydrolysis was also investigated and the best condition was achieved at the lowest total solids (5wt%) and the highest enzyme loading (85mg of Cellic CTec2 g(-1) of dry substrate). However, the highest concentration of fermentable sugars - 47.8 and 42.5gL(-1) for CGD and CGW, respectively - was obtained at 15wt% total solids using this same enzyme loading. Substrate hydrolysates had no inhibitory effects on the fermenting microorganism.

Insights into the effects of γ-irradiation on the microstructure, thermal stability and irradiation-derived degradation components of microcrystalline cellulose (MCC)

The microstructure, thermal stability and irradiated degradation components of microcrystalline cellulose were investigated under ⁶⁰Co γ-irradiation (0–1400 kGy).

Improvement of lignin yield and purity from corncob in the presence of steam explosion and liquid hot pressured alcohol

Non-food biomass such as corncob is a very abundant and promising feedstock for sustainable energy production in China.

Bioethanol from lignocellulosic biomass: current findings determine research priorities

“Second generation” bioethanol, with lignocellulose material as feedstock, is a promising alternative for first generation bioethanol. This paper provides an overview of the current status and reveals the bottlenecks that hamper its implementation. The current literature specifies a conversion of biomass to bioethanol of 30 to ~50% only. Novel processes increase the conversion yield to about 92% of the theoretical yield. New combined processes reduce both the number of operational steps and the production of inhibitors. Recent advances in genetically engineered microorganisms are promising for higher alcohol tolerance and conversion efficiency. By combining advanced systems and by intensive additional research to eliminate current bottlenecks, second generation bioethanol could surpass the traditional first generation processes.